Unlubricated plastic sleeve bearings are sometimes used as an alternative to expensive ball bearings. Perhaps the biggest advantage of plastic sleeve bearings is their remarkably low cost compared to roller bearings. There are, however, also performance advantages. Sleeve bearings require less radial space than roller bearings. They are better able to withstand shock conditions and have less difficulty with fatigue. Sleeve bearings are also much more quiet in operation than roller bearings.
The principal limitation in a sleeve bearing's performance is the so-called PV limit. PV is the product of load or pressure (P) and sliding velocity (V). A sleeve bearing subjected to increasing PV loading will eventually reach a point of failure known as the PV limit. For instance, high edge loading causes a sleeve bearing to reach its PV limit. The failure point is usually manifested by an abrupt increase in the wear rate of the bearing material.
As long as the mechanical strength of the bearing material is not exceeded, the temperature of the bearing surface is generally the most important factor in determining PV limit. Therefore, anything that affects surface temperature--coefficient of friction, thermal conductivity, lubrication, ambient temperature, running clearance, hardness and surface finish of mating materials--will also affect the PV limit of the bearing.
Thus, the first step in selecting and evaluating a sleeve bearing is determining the PV limit required by the intended application. It is usually prudent to allow a generous safety margin in determining PV limits, because real operating conditions often are more rigorous than experimental conditions
Determining the PV requirements of any application is a three step process First, the static loading per unit area (P) that the bearing must withstand in operation must be determined. For journal bearing configurations, the calculation is as follows: EQU P=W/(d.times.b)
P=pressure, psi(kg/cm.sup.2) PA1 W=static load, lb(kg) PA1 d=bearing surface ID, in.(cm) PA1 b=bearing length, in.(cm) PA1 V=surface velocity, in/min(cm/min) PA1 N=speed of rotation, rpm or cycles/min PA1 d=bearing surface ID, in.(cm)
Pressure (P) should not exceed certain maximum values at room temperature. These can be derived from a table of allowable static bearing pressure for most known materials. Next, the velocity (V) of the bearing relative to the mating surface must be calculated. For a journal bearing experiencing continuous rotation, as opposed to oscillatory motion, velocity is calculated as follows: EQU V=(dN)(.pi.)
where:
Finally, calculate PV as follows: EQU PV(psi-ft/min)=P(psi).times.V(in/min)12 or, in metric units: EQU PV(kg/cm.sup.2 -m/sec)=P(kg/cm.sup.2).times.V(cm/min)/6000
The PV limits of unlubricated bearing materials are generally available from the manufacturer of the material or from technical literature. Since PV limits for any material vary with different combinations of pressure and velocity as well as with other test conditions, it is prudent to consult the manufacturer for detailed information.
One material which is particularly well suited to bearing applications is the polyimide thermoset material sold by Hoechst Celanese under the trademark CELAZOLE.TM.. Properly lubricated CELAZOLE.TM. parts can withstand approximately 1 million psi-ft/min.
Despite the availability of new high performance materials such as CELAZOLE.TM. and the advantages sleeve bearings offer, sleeve bearings are still not used in many applications. This is believed to be due in part to the load limits of such bearings. Accordingly, there remains a need for improved sleeve bearings.